The subject invention is directed in general toward the vapor power cycle art and, more particularly, to an improved vapor power cycle capable of efficiently using low-grade heat sources.
The invention is particularly applicable to the use of unfocused solar energy and geothermal energy as low-grade heat sources and will be described with particular reference thereto; however, as will become appreciated, the invention is not limited to solar or geothermal energy and could be used with many different types of low-grade heat sources.
In the past, numerous vapor cycle engines have been devised to convert low-grade heat energy (heat energy available only at relatively low temperature) to useful mechanical power. A basic problem inherent with all of these engines is poor energy conversion efficiency. The poor conversion efficiency is due to the limited thermodynamic potential, or temperature difference, existing between the low-grade heat energy and any normal heat sink.
Examples of low-grade heat energy which can be made available for conversion to mechanical power are solar energy, geothermal energy, ocean temperature gradients, process waste heat; incinerator exhaust, electric lighting heat, the exhaust gases from gas turbine or internal combustion engines; and combustion of low-heating value fuels such as garbage, dung, and other wet organic matter. To utilize energy from these sources, the engine must be capable of absorbing and utilizing low-grade (relatively low temperature) heat. Generally, the higher the temperature of heat absorption, the higher the portion of absorbed heat that can be converted to useful work. The higher the temperature of heat absorption, the less of the low-grade heat that will be available for absorption. This is because of the necessity to effect heat transfer from the low-grade heat source to the engine. That portion of the low-grade heat which remains below the absorption temperature will not be transferred to the engine, and hence escapes and is wasted. It is, in effect, not available to the engine. In the case of solar energy, the escaped heat represents the heat losses from the solar collector, whereas for the other applicable examples, the escaped heat leaves with the exhausted gas or liquid. Thus, for most applications of low-grade heat utilization, an optimum total output can be found from the availability of the heat for absorption by the engine, and the efficiency of utilization by the engine. For most examples of low-grade heat utilization, absorption occurs at 150.degree.-300.degree. F.
The engine must subsequently reject the heat to a sink, which will usually be atmospheric air, or a cooling tower or surface water where available. The engine's heat rejection temperature will have to be maintained somewhat above the temperature of the sink medium to effect the transfer of heat. This means at least an 80.degree.-120.degree.F engine heat rejection temperature in nearly all cases. Consequently, the thermodynamic potential is small between most low-grade heat sources and the common heat sinks. Moreover, engine efficiency is approximately proportional to the thermodynamic potential. Hence, the efficiency of engines using the common forms of low-grade heat are generally less than half that obtained with the higher temperatures available from conventional fuels, such as, coal, oil or gas. Even where the low-grade heat is available without charge, the cost of owning and operating the low-grade heat engine can be significant due to larger size per unit output increasing the cost of capital and maintenance. Thus, it is desirable to increase the efficiency of utilization of low-grade heat so that a maximum amount of useful work can be obtained from a given engine. Further, increased efficiency of low-grade heat utilization will contribute to the overall conservation of energy.
Another major limitation on energy input temperature in many instances is the concurrent fluid and vapor pressure resulting in the engine. For the typical steam vapor cycle engine, system pressure are often limited by codes and ordinances. In many applications, 20 psig is the maximum allowable operating pressure. For previously-used steam vapor cycle engines, system pressure limitations effectively impose an energy input temperature limitation on the operating cycle. The 20 psig limit corresponds, of course, to a 258.degree.F limit on the temperature of heat input to the cycle.